In the process of the electrolysis of sodium chloride based on an ion exchange membrane method, the reduction of energy consumption is one of the most important issues. A detailed analysis of the voltage involved in the electrolysis of sodium chloride based on an ion exchange membrane method shows that the voltage includes, in addition to the theoretically required voltage, the voltage due to the ion exchange membrane, the overvoltages of the anode and cathode, and the voltage dependent on the distance between the anode and cathode in the electrolysis vessel.
As for the overvoltages of the electrodes among these various voltages, as far as the overvoltage of the anode is concerned, the so-called insoluble electrode referred to as the DSA (Dimension Stable Anode), provided with a coating layer of a platinum group oxide, reduces the overvoltage to a level lower than 50 mV or less such that no further improvement can be expected.
On the other hand, as far as the cathode is concerned, such conventionally used materials as mild steel, stainless steel and nickel exhibit overvoltages ranging from 300 to 400 mV. Accordingly, the activation of the surfaces of these materials has been studied for the purpose of reducing the overvoltages.
Examples include a highly active cathode produced from an oxide by thermal spraying of nickel oxide, cathodes utilizing Raney nickel based metals, cathodes taking advantage of composite plating with nickel and tin, and cathodes based on composite plating of active charcoal and oxides; all of these cathodes have been attempted to be applied to the cathode for use in hydrogen generation in caustic soda.
However, for the purpose of reducing the electrolysis voltage, it is necessary to further reduce the overvoltage, and accordingly electrodes based on various concepts have been proposed.
In JP-B-3-75635 (EP129734B), a layer of a heterogeneous mixture composed of a platinum group oxide and nickel oxide has been formed as a coating layer on a conductive metal base and thus a cathode having a low overvoltage has been made.
In U.S. Pat. No. 4,668,370, composite plating with a noble metal oxide and metallic nickel actualizes a low overvoltage and enhances the durability of the coating layer. In JP-B-6-33481 and JP-B-6-33492 (U.S. Pat. No. 4,900,419 or EP 298,055B), a composite substance composed of platinum and cerium is adopted as an electrode coating material, permitting enhancement of the poisoning resistance against iron.
In U.S. Pat. Nos. 5,645,930 and 5,882,723, ruthenium chloride, palladium chloride and ruthenium oxide are applied onto a conductive base, the base thus processed is calcined in the air, and thereafter subjected to electroless plating with nickel, thus improving the coating strength.
In JP-A-11-140680, an electrode coating layer mainly composed of ruthenium oxide is formed on a metal base, and further a porous and low-active protective layer is formed on the surface thereof, thus improving the electrode durability.
In JP-A-11-158678, an electrode coating layer is formed which is provided with a coating layer of ruthenium oxide, nickel and a rare earth metal capable of absorbing hydrogen formed on a metal base by pyrolysis, and thus electrolytic oxidation is prevented by maintaining the cathode at the hydrogen absorbing potential against the reverse current caused by the termination of the electrolysis.
In JP-A-11-229170, an electrodeposited nickel layer is provided in which ruthenium oxide is dispersed, the surface of the layer is coated with a conductive oxide composed of titanium oxide, and thus the resistance to mercury poisoning is improved.
However, even in the above described examples, the electrode operating life is short so that as matters stand, indeed, further elongation of the electrode operating life is an objective.
In WO01/28714, the interior of the coating layer is made porous and hence the surface area is made larger so that the resistance to the impurities found in alkali is improved, and a cathode having a low overvoltage is formed.